This invention generally relates to systems and methods for constructing holographic optical elements, and more particularly to systems and methods for automatically recording multi-hologram optical elements.
In the construction of holographic optical elements, a pair of collimated construction beams are projected so that they overlap, at a relative angle to each other, on a recording medium, producing an optical interference pattern that is recorded in the medium as an amplitude or phase distribution of closely spaced lines. Preferably, the construction beams are from the same source of coherent electromagnetic radiation, which, for example, may be a laser. The recording medium, as is well known, can be photographic emulsion, dichromated gelatin, a photopolymer, and the like, and can be coated or mounted on a suitable substrate such as a glass plate, a thin film, or the like.
Matched filters are one type of a holographic optical element that are used in optical correlator systems to detect the presence of a selected target in a scene or a field of view. To construct a matched filter, one of the construction beams, referred to as the signal beam, is spatially modulated by passing it through an image of the selected target, and then a lens performs an optical Fourier Transform operation on the modulated signal beam. The two construction beams then combine at the recording medium to produce a Fourier Transform hologram or matched filter, unique to the selected target. When the filter is used in a correlator system, a coherent light beam is passed through a view, which may include the selected target, Fourier transformed, and then transmitted through the matched filter and through an inverse transform lens. If the selected target is not present in the view, the output of the matched filter is relatively weak and diffused, and that output remains diffused as it passes through the inverse transform lens. However, if the suspected target is present in the submitted view, the light traversing the matched filter becomes collimated, and the inverse transform lens brings the output beam from the matched filter to a focus. A light sensitive detector is located at the focal point of the inverse transform lens; and when light of a sufficient intensity is focused on that detector, an output signal is produced. This output signal is used to trigger some type of device, which, depending on the apparatus in which the target recognition system is used, might be a simple alarm or a complex robotic guidance system, for example.
For many applications, it is beneficial to form multiple holograms on one recording medium. For example, because very little space is required on the matched filter plate in order to store a hologram, it is possible to construct a matched filter memory having an array of many holograms, which may represent different targets or different views of the same target. Such a matched filter memory is useful in an optical correlator system to identify a multitude of targets or to identify a target independent of the angle from which it is viewed. A matched filter memory having an array or library of holograms may be formed on one recording medium by presenting different views, one at a time, to the signal beam used to construct the filter, and changing the position on the recording medium exposed to that signal beam as the different views are presented to the signal beam.
Heretofore, optical systems have not been designed to automatically manufacture multiple holograms on individual recording mediums; and, instead, a significant amount of work by operating personnel, for example, to substitute one view for another or to change the position of the recording medium exposed to the modulated signal beam, has been required to form an array or overlap of holograms on a single recording medium. This necessary adjustment of various parts of the optical system is time consuming and often imprecise, especially so since the adjustments are normally done in the dark, or at best in very low levels of light, to minimize any undesirable exposure of the recording medium.